Sean Buzza’s eighth-minute try cancelled out Chris Johnson’s penalty, but Jack Elam went over for Lancashire to put them 10-7 up early on.
However, two tries in quick succession from Alex Ducker and Kyle Moyle saw Cornwall lead 21-10 after 20 minutes.
Moyle kicked a 33rd-minute penalty but Ashton Kassam-Sharples forced his way over from 15 metres to give Lancashire a lifeline with the half-time score at 24-17.
Lancashire drew the score level eight minutes into the second period when Sam Freeman went over, but will be frustrated not to have done better when Cornwall’s Josh Matavesi was sin-binned soon after.
Lancashire were held up over the line, but it was Cornwall who struck as Moyle kicked a 50th-minute penalty before a returning Matavesi extended the lead to 34-24 10 minutes later.
Any lingering doubts that Cornwall could add the Bill Beaumont Cup to their women’s Gill Burns Division Two success earlier in the day were ended when Max Bodilly ended a fine flowing move to cross for his side’s fifth try with 10 minutes left.
“Full credit to Lancashire, I thought they were outstanding, but I think all the plaudits go to our boys and our squad,” Cornwall captain Sam Matavesi told BBC Radio Cornwall.
“I think we handled the game so well when Josh went off. It probably gave us a bit of a spark that we needed, and when he came back on he made a few massive plays that helped in the win.
“It’s tough when you only get a few weeks’ prep; not everyone trains, not everyone takes part. So to get that result is massive.
“You can’t get bored of playing finals, especially at Twickenham, and to be in that contest to win is incredible.
“I’m proud of all the lads, all the coaches and all of Cornwall.”